A power amplifier is an electronic device which is used to increase the power of a signal in wireless transmitters, broadcast transmitters, and high fidelity audio equipment. The power of a signal is increased by taking power from a power supply and shaping the output signal to match the input signal.
The amplification factor, also called gain, is the extent to which an amplifier boosts the strength of a signal. The gain of an amplifier can be increased by cascading, that is, hooking up one amplifier after another.
One basic power amplifier system includes two stages. The first stage of amplification or gain can be from a field effect transistor (FET) that is set up to deliver its output power into a relatively high impedance at the operating frequency. The input of the second stage of amplification can be the gate of a large FET that has a very low input impedance. The amplifier system will include an inner stage-matching system to transform the low input impedance of the second stage to the high output impedance for the first stage. Often the first stage provides a higher gain than the second stage while the second stage is physically larger than the first stage to provide higher power.
A simple approach for achieving inner stage-matching is by using an inductor and capacitor combination. The inductor is connected to a voltage source at one end and to between the first stage and second stage amplifiers at the other end. The capacitor is connected in series between the inductor connection and the second stage amplifier. This configuration will cause the operating frequency to supply the correct impedance transformation. There will also be a certain phase shift of the signal that goes through the inner stage-matching, and it will only operate at a single frequency over a relatively narrow band. Further, selection of the components for the inductor and capacitor combination will be very critical.
Other approaches are used in many different types of electrical devices but have problems in certain applications. For example, when these amplifier systems are used in wireless communication devices, such as, but not limited to, cell phones, and connected to the cell phone antenna for transmitting and receiving signals, various non-linearities in the signal occur resulting in the power output of the cell phone antenna becoming highly non-linear resulting in unwanted interferring signals, the power output sometimes possibly dropping so no signal is transmitted, and the current requirement increasing to sometimes drain the cell phone batteries.
One reason for these problems is that the amplifier systems when connected to the cell phone antenna are designed to provide power for transmitting a signal into free space where the antenna load impedance to the signal is 50 ohms. When a cell phone user places the antenna near metallic objects such as the hood of a car, the roof, or the dashboard, this causes various reflective waves. The reflective waves change the impedance so the antenna load impedance is no longer 50 ohms but greatly increases or decreases. For a voltage standing wave ratio of 5 to 1, the antenna load impedance will range from about 10 ohms to about 250 ohms.
When the antenna load impedance starts to vary because of reflective waves, variations in gain will end up causing the various non-linearities and too much current consumption to occur. These non-linearities are particularly troublesome in a cell phone, because power broadcast into one cell phone channel will end up overflowing into adjacent channels. The power going into adjacent channels will ultimately limit the overall capacity of how many cell phone calls can be transmitted on a single frequency.
For any amplifier system to be used where major changes in load are possible, it is desirable to be able to have a linear power output. It should be noted that the amount of power to be delivered is less important than the fact that the power is provided linearly.
Further, since many modem electronic devices operate primarily on battery power, it is desirable to have a linear power output with the maximum power output itself lower than a fixed level when the load is variable. When the maximum power output is low, the current drain from the battery is minimized, and therefore the battery can maintain its voltage.
Also, while the load is varying, it is also desirable to be assured of having a minimum power output to assure continued operation of the electronic device.
Another approach has been to have a balanced power amplifier system, which uses two parallel lines with splitters, phase shifters, and combiners to balance each other to compensate for variable loads. Unfortunately, the additional components not only increase cost but physical size, but also impose additional power consuming components which more quickly drain the cell phone batteries. This power loss is incurred even when the antenna load impedance is constant at 50 ohms.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.